Conformers Research Articles

Iodine Gauche Effect Induced by an Intramolecular Hydrogen Bond.

The gauche conformer in 1-X,2-Y-disubstituted ethanes, that is, the staggered orientation in which X and Y are in closer contact, is only favored for relatively small substituents that do not give rise to large X···Y steric repulsion. For more diffuse substituents, weakly attractive orbital interactions between antiperiplanar bonds (i.e., hyperconjugation) cannot overrule the repulsive forces between X and Y. Our quantum chemical analyses of the rotational isomerism of XCH2CH2Y (X = F, OH; Y = I) at ZORA-BP86-D3(BJ)/QZ4P reveal that indeed the anti conformer is generally favored due to a less destabilizing I···F and I···O-H steric repulsion. The only case when the gauche conformer is preferred is when the hydroxyl hydrogen is oriented toward the iodine atom in the 2-iodoethanol. This is because of the significantly stabilizing covalent component of the I···H-O intramolecular hydrogen bond. Therefore, we show that strong intramolecular interactions can overcome the steric repulsion between bulky substituents in 1,2-disubstituted ethanes and cause the gauche effect. Our quantum chemical computations have guided nuclear magnetic resonance experiments that confirm the increase in the gauche population as X goes from F to OH.

Quantum Calculations on a New CCSD(T) Machine-Learned Potential Energy Surface Reveal the Leaky Nature of Gas-Phase Trans and Gauche Ethanol Conformers.

Ethanol is a molecule of fundamental interest in combustion, astrochemistry, and condensed phase as a solvent. It is characterized by two methyl rotors and trans (anti) and gauche conformers, which are known to be very close in energy. Here we show that based on rigorous quantum calculations of the vibrational zero-point state, using a new ab initio potential energy surface (PES), the ground state resembles the trans conformer, but substantial delocalization to the gauche conformer is present. This explains experimental issues about identification and isolation of the two conformers. This “leak” effect is partially quenched when deuterating the OH group, which further demonstrates the need for a quantum mechanical approach. Diffusion Monte Carlo and full-dimensional semiclassical dynamics calculations are employed. The new PES is obtained by means of a Δ-machine learning approach starting from a pre-existing low level density functional theory surface. This surface is brought to the CCSD(T) level of theory using a relatively small number of ab initio CCSD(T) energies. Agreement between the corrected PES and direct ab initio results for standard tests is excellent. One- and two-dimensional discrete variable representation calculations focusing on the trans–gauche torsional motion are also reported, in reasonable agreement with experiment.

Conformer Selection Upon Dilution with Water: The Fascinating Case of Liquid Ethylene Glycol Studied via Molecular Dynamics Simulations.

The aqueous solution of ethylene glycol (EG) is a binary liquid mixture that displays rich conformational and structural behaviour, which has not yet been adequately explored through atomistic molecular dynamics simulations. Herein, employing an accurate force field for EG, several physical properties of this solution are calculated to be in quantitative agreement with experimental data. While 79 % of molecules in neat liquid EG exist with their central OCCO dihedral in the gauche state, this fraction increases to 89 % in the dilute aqueous solution, largely in response to the increase in the static dielectric constant of the solution from that of neat liquid EG. The increase in gauche conformers increases the mean dipole moment of EG molecules in the solution which is additionally contributed by specific conformational states of the two terminal HOCC dihedral angles.

Synthesis and crystal structure of trans-di-aqua(1,4,8,11-tetra-aza-undeca-ne)copper(II) isophthalate monohydrate.

In the title hydrated mol-ecular salt, [Cu(C7H20N4)(H2O)2](C8H4O4)·H2O, the metal ion is coordinated by the two primary and two secondary N atoms of the amine ligand and the mutually trans O atoms of the water mol-ecules in a tetra-gonally distorted octa-hedral geometry. The average equatorial Cu-N bond lengths (2.013 and 2.026 Å for Cu-Nprim and Cu-Nsec, respectively) are substanti-ally shorter than the average axial Cu-O bond length (2.518 Å). The tetra-amine ligand adopts its energetically favored conformation with its five- and six-membered chelate rings in gauche and chair conformations, respectively. In the crystal, the N-H donor groups of the tetra-amine, the acceptor carboxyl-ate groups of the isophthalate dianion and both the coordinated water mol-ecules and the water mol-ecule of crystallization are involved in numerous N-H⋯O and O-H⋯O hydrogen bonds, resulting in the formation of electroneutral layers oriented parallel to the ac plane.

Model phospholipid interaction with cholesterol and melatonin: Raman spectroscopy and density functional theory study

AbstractWe present a Raman spectroscopy and quantum chemical computational study for model phospholipid membrane molecule dipalmitoylphosphatidylcholine (DPPC) mixed with cholesterol or melatonin at various concentrations. Utilizing a confocal Raman microspectroscopy setup, we capture a DPPC conformation state by evaluating the number of trans/gauche bonds using I1064/I1090 and I1128/I1090 vibrational mode intensity ratios. We show that the cholesterol increases the number of trans bonds in DPPC hydrocarbon chains, that is, causing membrane ordering, while melatonin addition introduces gauche conformers, generating disorder. It is noticed that the I1128/I1090 intensity ratio is a more suitable parameter for the characterization of DPPC hydrocarbon chain structural properties compared with I1064/I1090. We thus clarify our experimental observations by performing careful density functional theory (DFT) calculations of Raman spectra for DPPC tails, containing up to three gauche bonds at different chain positions. The obtained results demonstrate the strong interconnection between 1064 and 1090 cm−1 vibrational modes originating from their complex splitting and shifting along with continuous Raman activity redistribution upon various gauche bonds introduction, while it is not the case for 1128 and 1090 cm−1 modes pair.

Structure transition of aliphatic m,6-Polyurethane during heating investigated using in-situ WAXS, SAXS, and FTIR

Thermoplastic polyurethanes (TPU) produced in the reaction of hexamethylene diisocyanates (HDI) and diols are widely applied in industry. The phase transition behavior of the HDI-diol hard domain was investigated as a structural analog to conventional polyamides (PA), wherein an oxygen atom substituted a methylene group. This substitution brought about a distinct phase transition behavior; the apparent merging of two diffraction peaks into the intermediate peak was not observed; upon heating, the methylene groups orderly packed in the crystalline domain in a trans conformation, gradually transforming into a disorderly gauche conformation was also absent. Such a transition observed in this work is not in accordance with the Brill transition no matter that it is defined by apparent experimental criterion or essential molecular fundamental. The melting sequences of the phase transition, i) mesophase; ii) crystallites; iii) blocks mixing, were jointly revealed by multiple in-situ techniques, WAXS, FTIR and SAXS.

Hydrogen bonds in aqueous choline chloride solutions by DFT calculations and X-ray scattering

Choline chloride (ChCl) is a common hydrogen bond acceptor (HBA) for deep eutectic solvents (DESs), and the hydrogen bonds between ChCl and H2O impact the physicochemical properties of DES-water mixtures. In the present work, the hydrogen bonds in aqueous ChCl solutions were investigated combining with Raman spectroscopy, density functional theory (DFT) calculations, X-ray scattering and empirical potential structure refinement (EPSR). The Ch+ in aqueous ChCl solutions mainly exists as the gauche conformation. The aqueous ChCl solutions were found to be stabilized by both classical hydrogen bonds (O-H⋯Y, Y = Cl, OW) and carbon-hydrogen bonds (C-H⋯Y, Y = OW) between the components. The orientation of the hydrogen bonds is close to linear (∼180°), while the orientation of the carbon-hydrogen bonds is relatively weak (150°∼176°). The existence of carbon-hydrogen bonds elucidates the high coordination of quaternary ammonium group with H2O. Carbon-hydrogen bonds are found in the partial radial distribution functions, the angle distribution functions and the atoms in molecules analysis. Hydrogen bonds and carbon-hydrogen bonds can affect the physicochemical properties of aqueous ChCl solutions and DES-H2O mixtures. This fundamental understanding of the aqueous ChCl solutions will enable future developments in functional design and application of choline-based DESs.

L-Histidine with nitric acid: A comparison of crystal structures and Hirshfeld surfaces analysis

L-histidine crystallizes in three different crystal structure with nitric acid, i.e. L-histidinium nitrate, L-histidinium nitrate monohydrate and L-Histidinium dinitrate. Also there is DL-histidinium dinitrate. Though the structural aspects of histidinium nitrate monohydrate, L-Histidinium dinitrate and DL-histidinium nitrate are well studied, the structural studies on L-histidinium nitrate is not dealt in detail. Here we have tried to discuss in detail the crystal structure of L-histidinium nitrate using both single crystal X-ray and neutron diffraction. The crystal structure consists one L-histidine molecule which has the amino and imino groups protonated, but the carboxylic acid group is deprotonated. The nitrate ion is in anionic state ie NO3−. The conformation of histidine (N1-C2-C3-C4=64.99°) is gauche (-). Also a comparison of the structure of L-histidinium nitrate with other three available crystal structures retrieved using CSD have been carried out. It was found that L-histidine in L-histidinium nitrate monohydrate adopts gauche (+) conformation and in other three structure it is gauche (-), and L-histidinium nitrate has the lowest volume. Hirshfeld surface and interaction energies analysis of all the above structures were carried out using the x-ray data. L-histidinium nitrate monohydrate shows maximum globuarity. Interaction energy obtained from energy framework showed that L-histidinium nitrate monohydrate has maximum total energy of -41.5(kJ/Mole). The asphericity of our structure L-histidinium nitrate is 0.252 indicating prolate nature unlike the other three complexes. The percentage of H…H contacts is maximum in L-histidinium nitrate which is 27.3%. A comparison of various contacts of histidine between the x-ray and neutron structure of ours shows that the maximum change is reflected in the C…H and H…H contacts with C…H having a higher percentage and H…H having a lower percentage in the neutron structure indicating that the neutron diffraction maps shows the hydrogen bond interaction more accurately.The three complexes, except DL Histidinium dinitrate crystallizes in a non-centrosymmetric space group and hence could exhibit second-order optical non-linear properties, attempts were made to calculate heats of formation, dipole moments (D), polarizabilities (α) and first hyperpolarizabilities (β) using the semi empirical method at the PM6 level using MOPAC2009 and the packing energy as implemented in MERCURY. It was found that the average β of L-histidinium nitrate monohydrate was the highest amongst the three showing that it is more suitable for NLO material.

Influence of Aliphatic Chain Length on Structural, Thermal and Electrochemical Properties of n-alkylene Benzyl Alcohols: A Study of the Odd-Even Effect.

The century-old, well-known odd–even effect phenomenon is still a very attractive and intriguing topic in supramolecular and nano-scale organic chemistry. As a part of our continuous efforts in the study of supramolecular chemistry, we have prepared three novel aromatic alcohols (1,2-bis[2-(hydroxymethyl)phenoxy]butylene (Do4OH), 1,2-bis[2-(hydroxymethyl)phenoxy]pentylene (Do5OH) and 1,2-bis[2-(hydroxymethyl)phenoxy]hexylene (Do6OH)) and determined their crystal and molecular structures by single-crystal X-ray diffraction. In all compounds, two benzyl alcohol groups are linked by an aliphatic chain of different lengths (CH2)n; n = 4, 5 and 6. The major differences in the molecular structures were found in the overall planarity of the molecules and the conformation of the aliphatic chain. Molecules with an even number of CH2 groups tend to be planar with an all-trans conformation of the aliphatic chain, while the odd-numbered molecule is non-planar, with partial gauche conformation. A direct consequence of these structural differences is visible in the melting points—odd-numbered compounds of a particular series display systematically lower melting points. Crystal and molecular structures were additionally studied by the theoretical calculations and the melting points were correlated with packing density and the number of CH2 groups. The results have shown that the generally accepted rule, higher density = higher stability = higher melting point, could not be applied to these compounds. It was found that the denser packaging causes an increase in the percentage of repulsive H‧‧‧H interactions, thereby reducing the stability of the crystal, and consequently, the melting points. Another interesting consequence of different molecular structures is their electrochemical and antioxidative properties—a non-planar structure displays the highest oxidation peak of hydroxyl groups and moderate antioxidant activity.

A highly polar hybrid perovskite of [FCH2CH2NH3][Mn(HCOO)3

AbstractA new hybrid perovskite of [FCH2CH2NH3][Mn(HCOO)3] has been successfully prepared by utilizing the ammonium of FCH2CH2NH3+, and it is the first example incorporating F‐substituted ammonium in the class of ammonium metal formate framework. The compound crystallizes in polar, orthorhombic space group Pna21, and the cell parameters are a=8.5901(3)–8.7158(6), b=8.4613(3)–8.5216(6), c=12.4837(4)–12.6106(9) Å and V=907.36(5)–936.6(1) Å3 from 80 K up to 410 K just before the decomposition, showing positive thermal expansions in all directions and no structural phase transition in the temperature region. The FCH2CH2NH3+ cation in the cavity of the NaCl‐like Mn‐formate framework adopts N−C−C−F gauche conformation, and its NH3+ and F ends are anchored to the anionic framework by N−H⋅⋅⋅O and C−H⋅⋅⋅F H‐bonds. The large dislocation of the NH3+ end from the anionic framework, about 1.4 Å, results in an estimated high electric polarization about 10 μCcm−2 for the polar structure. The material is also a spin‐canted antiferromagnet with the Néel temperature of 8.3 K.

Experimental/Computational Study on the Impact of Fluorine on the Structure and Noncovalent Interactions in the Monohydrated Cluster of ortho-Fluorinated 2-Phenylethylamine.

Fluorine-containing medicinal compounds frequently allow modulation of physical-chemical properties. Here, we address the effect of fluorine, near the ethylamino side chain, on conformational flexibility and noncovalent interactions (NCIs) of the selected jet-cooled monohydrated cluster of 2-(2-fluoro-phenyl)-ethylamine (2-FPEA) by mass-selected resonance-enhanced two-photon ionization and ionization-loss stimulated Raman spectroscopies. Our results show that Raman spectral signatures of the 2-FPEA-H2O cluster match the scaled harmonic vibrational Raman frequencies, resulting from density functional theory calculations of the most stable 2-FPEA gauche conformer hydrogen-bonded (HB) to water, confirming the three-dimensional cluster structure. This predicted electronic structure, together with NCI analysis, allows visualization and assessment of the attractive and repulsive interactions. The comparison of the NCIs and revealed red (O-H and N-H stretches) and blue shifts (C-H stretches and CH2 out-of-plane bends) of the cluster to other class members confirm O-H···N, N-H···π, C-H···O, and C-H···F HB formation and their contribution to structure stabilization, uncovering the potential of the approach.

Poly(cyclosilane) Connectivity Tunes Optical Absorbance.

We report herein the influence of skeletal connectivity on the conformation-dependent optical properties of cyclosilane homo- and copolymers. 1,3-Linked cyclosilanes were bathochromically shifted by 20 nm in solution relative to 1,4-linked cyclosilanes, an effect reproduced by quantum chemical calculations on oligomeric model systems. Polysilane optical properties are conformation-dependent, and 1,3-linked cyclosilanes were hypothesized to adopt a favorable conformation unavailable to 1,4-linked cyclosilanes constrained to an endocyclic gauche conformation. Copolymerization of the isomeric cyclosilanes 1,3Si6 and 1,4Si6 afforded linear statistical copolymers, as characterized by 1H and 29Si NMR spectroscopies. The distinct connectivity of each comonomer was found to give rise to tunable absorption spectra, where the position of the absorption band systematically increased with the increased corporation of 1,3Si6. Computational studies pointed to conformation-dependent changes in orbital symmetry in shifting the most intense transition from the low-energy highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition to a higher-energy HOMO → LUMO + n transition. The results of these studies demonstrate for the first time the role of silicon skeletal connectivity in controlling conformation and optoelectronic properties and provide new insight into the structure-based design of solution-processable silicon-based polymeric materials.

Interaction Mechanisms and Interface Configuration of Cysteine Adsorbed on Gold, Silver, and Copper Nanoparticles.

Cysteine-protected metal nanoparticles (NPs) have shown interesting physicochemical properties of potential utility in biomedical applications and in the understanding of protein folding. Herein, cysteine interaction with gold, silver, and copper NPs is characterized by Raman spectroscopy and density functional theory calculations to elucidate the molecular conformation and adsorption sites for each metal. The experimental analysis of Raman spectra upon adsorption with respect to free cysteine indicates that while the C-S bond and carboxyl group are similarly affected by adsorption on the three metal NPs, the amino group is sterically influenced by the electronegativity of each metal, causing a greater modification in the case of gold NPs. A theoretical approach that takes into consideration intermolecular interactions using two cysteine molecules is proposed using a S-metal-S interface motif anchored to the metal surface. These interactions generate the stabilization of an organo-metallic complex that combines gauche (PH) and anti (PC) rotameric conformers of cysteine on the surface of all three metals. Similarities between the calculated Raman spectra and experimental data confirm the thiol and carboxyl as adsorption groups for gold, silver, and copper NPs and suggest the formation of monomeric "staple motifs" that have been found in the protecting monolayer of atomic-precise thiolate-capped metal nanoclusters.

Effect of heat treatment with alternating tension on microstructure and elastic recovery of poly(trimethylene terephthalate) fibers

AbstractPoly(trimethylene terephthalate) (PTT) fibers have drawn much attention due to their excellent elastic recovery. Currently, PTT fibers with higher elasticity are in high demand in some special applications. In this article, in order to further improve the elasticity of PTT fibers, heat treatment with alternating tension and relaxation (AlterTR) was carried out, and as a comparison, heat treatments under the conditions of tension or relaxation were performed simultaneously. The relationships of conformation transition, crystallinity, and orientation with elastic recovery of heat treated PTT fibers were investigated. With the increase of crystallinity during the treatment, part of the higher‐energy trans conformation of the heat treated PTT fibers was converted to a lower‐energy gauche conformation, which is a primary cause of elastic recovery. AlterTR heat treated PTT fibers had both high crystallinity and high orientation, which resulted in a high elastic recovery of 79% at 30% fixed elongation after 5 cycles of stretching testing, 7% higher than untreated PTT fibers. The elastic recovery was also obviously higher than the PTT fibers heat treated under the conditions of tension or relaxation.

The synergetic and multifaceted nature of carbon–carbon rotation reveals the origin of conformational barrier heights with bulky alkane groups

AbstractDesigning compounds with as long carbon–carbon bond distances as possible to challenge conventional chemical wisdom is of current interest in the literature. These compounds with exceedingly long bond lengths are commonly believed to be stabilized by dispersion interactions. In this work, we build nine dimeric models with varying sizes of alkyl groups, let the carbon–carbon bond flexibly rotate, and then analyze rotation barriers with energy decomposition and information‐theoretic approaches in density functional theory. Our results show that these rotations lead to extraordinarily elongated carbon–carbon bond distances and rotation barriers are synergetic and multifaceted in nature. The dominant factor contributing to the relative stability of the dimers with bulky alkane groups is not the dispersion force but the electrostatic interaction with steric and exchange‐correlation effects playing minor yet indispensable roles.

Investigation of menopause-induced changes on hair by Raman spectroscopy and chemometrics

The ending of estrogen production in the ovaries after menopause results in a series of important physiologic changes, including hair texture and growth. In this study we demonstrate that Raman spectroscopy can be used successfully as a tool to probe menopause-induced changes on hair, in particular when coupled with suitable chemometrics approaches.The detailed analysis of the average Raman spectra (in particular of the Amide I and νS-S stretching spectral regions) of the hair samples of women pre- and post-menopause allowed to estimate that absence of estrogen in post-menopause women leads to an average reduction of ∼12% in the thickness of the hair cuticle, compared to that of pre-menopause women, and revealed the strong prevalence of disulphide bonds in the most stable gauche–gauche–gauche conformation in the hair cuticle. From the analysis of the νS-S stretching spectral region it could also be concluded that the amount of α-helix keratin is slightly higher for post-menopause than for pre-menopause women.A series of statistical models were developed in order to classify the hair samples. Outperforming the traditional PCA-LDA (principal component analysis – linear discriminant analysis) approach, in the present study a GA-LDA (genetic algorithm – linear discriminant analysis) strategy was used for variable reduction/selection and samples’ classification. This strategy allowed to develop of a statistical model (L16), which has exceptional prediction capability (total accuracy of 96.6%, with excellent sensitivity and selectivity) and can be used as an efficient instrument for the hair samples’ classification.In addition, a new chemometrics approach is here presented, which allows to overcome the intrinsic limitations of the GA algorithm and that can be used to develop statistical models that use GA as the variable reduction/selection method, but superseding its stochastic nature. Three suitable models for classification of the hair samples according to the menopause status of the women were developed using this novel approach (LV17, BLV20 and PLS7 models), which are based on the Fisher’s and Bayers’ LDA approaches and the PLS-DA method. The followed new chemometrics approach uses the results of a large set of GA-LDA runs over the full data matrix for the selection of the reduced data matrices. The criterion for the selection of the variables is their statistical significance in terms of number of occurrences as solutions of the whole set of GA-LDA runs.

Rotational spectrum of anti- and gauche-4-cyano-1-butyne (C5H5N) – An open-chain isomer of pyridine

The analysis of the rotational spectrum of 4-cyano-1-butyne, an open-chain isomer of pyridine, is presented herein for the first time. Transitions for the ground vibrational states of both conformational isomers, anti and gauche, were observed in the 8–38 GHz and 130–360 GHz frequency ranges and fit to effective sextic distorted-rotor Hamiltonians with low uncertainty (σfit < 50 kHz), allowing for their potential detection by radioastronomy or in harsh reaction environments. The microwave region includes hyperfine-split transitions for both conformers, allowing for the experimental determination of the χaa, χbb, and χcc quadrupole coupling constants. In the millimeter-wave region, not every observable ground-state transition could be satisfactorily addressed by a single-state Hamiltonian due to Coriolis coupling with the low-energy vibrationally excited states. Thus, effective single-state fits of the ground states were obtained by including only Ka-series transitions without obviously perturbed energy levels. Importantly, the effective fit does include the most intense transitions throughout the observed frequency ranges. The predicted values (MP2/cc-pVTZ) are in close agreement with the experimental spectroscopic constants. These data provide a sufficient foundation for the identification of anti- and gauche-4-cyano-1-butyne in the interstellar medium by radioastronomy. The gauche conformer is estimated to lie 1.00 (5) kcal/mol higher in energy than the anti conformer, as determined using experimental transition intensities.

Conformational Selection in Biocatalytic Plastic Degradation by PETase

Due to the steric effects imposed by bulky polymers, the formation of catalytically competent enzyme and substrate conformations is critical in the biodegradation of plastics. In poly(ethylene terephthalate) (PET), the backbone adopts different conformations, gauche and trans, coexisting to different extents in amorphous and crystalline regions. However, which conformation is susceptible to biodegradation and the extent of enzyme and substrate conformational changes required for expedient catalysis remain poorly understood. To overcome this obstacle, we utilized molecular dynamics simulations, docking, and enzyme engineering in concert with high-resolution microscopy imaging and solid-state nuclear magnetic resonance (NMR) to demonstrate the importance of conformational selection in biocatalytic plastic hydrolysis. Our results demonstrate how single-amino acid substitutions in Ideonella sakaiensis PETase can alter its conformational landscape, significantly affecting the relative abundance of productive ground-state structures ready to bind discrete substrate conformers. We experimentally show how an enzyme binds to plastic and provide a model for key residues involved in the recognition of gauche and trans conformations supported by in silico simulations. We demonstrate how enzyme engineering can be used to create a trans-selective variant, resulting in higher activity when combined with an all-trans PET-derived oligomeric substrate, stemming from both increased accessibility and conformational preference. Our work cements the importance of matching enzyme and substrate conformations in plastic hydrolysis, and we show that also the noncanonical trans conformation in PET is conducive for degradation. Understanding the contribution of enzyme and substrate conformations to biocatalytic plastic degradation could facilitate the generation of designer enzymes with increased performance.

Ionic Association in CH3-(CH2-CF2) n -CH3(PVDF)-Li+-(CF3SO2)2N- for n = 1, 4: A Computational Approach.

The ionic conductivity of solid polymer electrolytes is governed by the ionic association caused by the polymer···Li+ and the anion···Li+ interactions. We performed the density functional calculation to analyze the molecular interactions in the CH3–(CH2–CF2)n–CH3–Li+–(CF3SO2)2N– for n = 1,4 systems. The gauche conformation is predicted in the lowest energy conformer of pure polymer except for n = 1. The lithium coordination number with the polymer is changed from 3 to 2 in the presence of anion for n = 2, 4 systems. The consequences of the Li+ ion and Li+–(CF3SO2)2N– to the vibrational spectrum are studied to understand the ionic association at the molecular level.

Morphological Behavior of DPPC-DMPC Two-dimensional Mixed Monolayer on Water Surface by Dropping Method

We have investigated the morphology of dipalmitoyl phosphatidyl choline (DPPC)-dimyristoyl PC (DMPC) mixed monolayer formed on the water surface by the dropping method using surface tension measurement (STm), Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IERS). The limiting molecular area (A0) obtained by STm showed that the negative deviation at &lt; mole fraction (xDMPC)= 0.4 whereas the positive deviation at &gt; xDMPC =0.6 when DMPC was added to the DPPC monolayer. BAM images also showed that the condensed rigid monolayer at xDMPC &lt; 0.4 changed to the expandable monolayer at xDMPC &gt; 0.5. Moreover, IER spectra showed that the alkyl chains of both DPPC and DMPC molecules are in trans conformation in mixed monolayer at xDMPC =0.2 whereas gauche conformation existed at xDMPC = 0.8. Above findings have justified the transition of flexible gauche conformation alkyl chains of DMPC molecule into rigid trans conformation by the addition of DPPC molecule at xDMPC &lt; 0.4 whereas alkyl chains retained flexible gauche conformation in both DPPC and DMPC molecules at xDMPC &gt; 0.6.